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Ab Initio Assessment of the Bonding in Disulfonates Containing Divalent Nitrogen andPhosphorus Atoms

Andersen, Vinca Bonde; Berg, Rolf W.; Shim, Irene

Published in:ACS Omega

Link to article, DOI:10.1021/acsomega.7b00266

Publication date:2017

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Andersen, V. B., Berg, R. W., & Shim, I. (2017). Ab Initio Assessment of the Bonding in Disulfonates ContainingDivalent Nitrogen and Phosphorus Atoms. ACS Omega, 2(8), 4447-4455.https://doi.org/10.1021/acsomega.7b00266

Ab Initio Assessment of the Bonding in Disulfonates ContainingDivalent Nitrogen and Phosphorus AtomsVinca B. Andersen, Rolf W. Berg, and Irene Shim*

Department of Chemistry, Technical University of Denmark, DTU building 207, DK-2800 Kgs. Lyngby, Denmark

ABSTRACT: The iminodisulfonate, [N(SO3)2]3−, and phosphinodisulfonate,

[P(SO3)2]3−, ions have been investigated by performing ab initio MP2/6-

311+G** calculations. The nitrogen and phosphorus atoms as part of the ionsare shown to be divalent with a negative charge and two lone pairs on thenitrogen and phosphorus atoms. The experimentally known calcium sodiumiminodisulfonate trihydrate and the analogous unknown compound calciumsodium phosphinodisulfonate trihydrate have also been investigated using theMP2/6-311+G** calculations. For the nitrogen compound, only minorchanges occur in the iminodisulfonate ion when it becomes part of the calciumsodium iminodisulfonate trihydrate. For the phosphorus compound, thegeometry of the phosphinodisulfonate ion changes significantly as part ofcalcium sodium phosphinodisulfonate trihydrate. Furthermore, the chargesassociated with the atoms in calcium sodium phosphinodisulfonate trihydrateare quite different from those of the phosphinodisulfonate ion. For calciumsodium iminodisulfonate trihydrate, the Raman spectrum has been measured, and it compares well with the spectrum derivedusing HF/6-311+G** calculations.

1. INTRODUCTIONAt the Enstedvaerk power plant, Aabenraa, Denmark, someinsoluble crystals were discovered in the water pipelines of theflue gas cleaning systems removing sulfur dioxide. In 2002,Fogh et al. carried out X-ray diffraction investigations of thesecrystals, and the crystalline compound was identified as calciumsodium iminodisulfonate trihydrate, CaNa[N(SO3)2]·3H2O.

1

In 2003, Rasmussen et al.2 published powder diffraction datafor the same compound.The [N(SO3)2]

3− and [NH(SO3)2]2− anions have been

prepared in combination with cations potassium, sodium, andcalcium by several groups.3−7 Calcium sodium iminodisulfonatewas first prepared in 1892 by Divers and Haga, as described intheir study on imidosulphonates where they suggested thename imidosulphonate.5 In 1896, Divers and Haga published acomprehensive review of a previous work on severalimidosulphonate salts.6

In 1956, Jeffrey and Jones analyzed the crystal structure ofthe potassium salt of the aminodisulphonate ion, [NH-(SO3)2]

2−, by X-ray diffraction.8 The focus was on measuringthe S−N bond length and establishing the configuration ofnitrogen. They determined the S−N bond length to be 1.655 Å,which they interpreted as a partial double bond between S anda sp2-hybridized N atom. In addition, Cruickshank9 has alsosuggested that S−N has a π bond character. Cruickshank andJones10 refined the crystal structure of potassium iminodi-sulphonate based on the data from Jeffrey and Jones.8 Theirresults essentially confirmed the results obtained by Jeffrey andJones.8 Hodgson et al.11 tried to locate the hydrogen in[NH(SO3)2]

2− and to establish the configuration around thenitrogen atom by neutron diffraction. The structure was refined

to give 4-fold bonding to two sulfur atoms. Furthermore, thestructure was refined with two half hydrogen atoms about thetwofold axis; therefore, they rejected trigonal bonding fornitrogen. Barbier et al.12 carried out X-ray diffraction studies ofK3[N(SO3)2]·H2O and K2[NH(SO3)2]. Their results for[NH(SO3)2]

2− confirmed the structure obtained by Cruick-shank and Jones10 and by Hodgson et al.11 Barbier et al.12 alsodetermined the mean bond length between N and S inK2[N(SO3)2]·H2O to be 1.606 ± 0.002 Å and the mean bondangle S−N−S to be 120.83 ± 0.11°. Hall et al.13 supplementedtheir X-ray diffraction measurements of K3[N(SO3)2]·H2O withIR and Raman spectroscopy. They observed that the S−N bondlengths in [N(SO3)2]

3−, 1.609 ± 0.002 Å, are markedly shorterthan those of the S−N bonds in compounds containing[NH(SO3)2]

2−.In the present work, the [N(SO3)2]

3− ion is investigated bycarrying out ab initio Hartree−Fock (HF) calculations in theRoothaan formalism14 and also by performing second-orderMøller−Plesset (MP2)15 calculations. In addition, the analo-gous phosphinodisulfonate ion, [P(SO3)2]

3−, is also inves-tigated by performing HF and MP2 calculations. The initialstructure of [N(SO3)2]

3− is based upon the structure obtainedby Fogh et al.1 The geometry of the investigated ions wasoptimized using MP2/6-311+G** calculations, and the resultselucidate the bonding properties of the ions. Furthermore, theelectrostatic charges associated with the atoms in [N(SO3)2]

3−

and [P(SO3)2]3− were determined on the basis of the

Received: March 7, 2017Accepted: May 19, 2017Published: August 11, 2017

Article

http://pubs.acs.org/journal/acsodf

© 2017 American Chemical Society 4447 DOI: 10.1021/acsomega.7b00266ACS Omega 2017, 2, 4447−4455

This is an open access article published under an ACS AuthorChoice License, which permitscopying and redistribution of the article or any adaptations for non-commercial purposes.

optimized structures, and the results are compared with thestandard Lewis structures of the ions.In addition, ab initio MP2/6-311+G** studies were

performed on calcium sodium iminodisulfonate trihydrate andfor the analogous calcium sodium phosphinodisulfonatetrihydrate. The bond lengths and angles determined by theab initio calculations are compared to the known experimentaldata. The location of hydrogen and the configuration ofnitrogen in [NH(SO3)2]

2− were elucidated by the MP2/6-311+G** calculation.The Raman spectrum has been measured for the calcium

sodium iminodisulfonate trihydrate salt obtained from theEnstedvaerk power plant, Denmark. In addition, Raman spectrawere obtained for [N(SO3)2]

3− and [P(SO3)2]3−, as well as for

CaNa[N(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O at the HF/6-311+G** level. The calculated normal modes for [N-(SO3)2]

3− and CaNa[N(SO3)2]·3H2O are used to assign theexperimental spectra.

2. RESULTS AND DISCUSSION2.1. Ion [NH(SO3)2]

2−. The crystal structure of potassiumaminodisulphonate, K2[NH(SO3)2], has been studied exper-imentally by other authors.8,10,11 The S−N bond lengths rangefrom 1.6558 to 1.674 Å.11 In refs 8−10, the location of the Hatom could not be determined, but it was suggested that thegeometry around the N atom is trigonal planar, suggesting a sp2

hybridization of nitrogen. The results by Hodgson et al.11 andby Barbier et al.12 indicated that the N atom is sp3-hybridized.The geometry of [NH(SO3)2]

2− has been optimized at theMP2/6-311+G** level. The molecule was initially built withS1−N1−H1−S2 in a plane.Figure 1 shows the optimized structure of [NH(SO3)2]

2−.The total energy for [NH(SO3)2]

2− is −1300.95739 au.

The optimized structure of [NH(SO3)2]2− has C1 symmetry.

The hydrogen atom is no longer in the plane containing S1−N1−S2, and nitrogen can be regarded as sp3-hybridized. Thestructural parameters obtained in the MP2/6-311+G**calculations for [NH(SO3)2]

2− are listed in Table 1 togetherwith the electrostatic charges determined for each atom.The calculated average S−N bond length, 1.738 Å, is larger

than the experimentally determined values from refs 8, 10, 11by a magnitude ranging from 0.064 to 0.083 Å. The average S−O bond length, 1.478 Å, derived in the present work is largerthan that from refs 8, 10, 11 by 0.025−0.036 Å. Thesediscrepancies are presumably related to the exclusion of thecations in the model. The H1−N1−S1 and H1−N1−S2 anglesare reasonably close to the tetrahedral angle, 109.47°, expectedfor a sp3-hybridized N atom, whereas the S1−N1−S2 angle,127.82°, is significantly larger than the tetrahedral angle. This isprobably due to the bulky SO3 groups. The dihedral angle O1−S1−S2−O5 is 33.27°. Thus, the results derived in the presentwork clearly show that the arrangement around the N atom isnot planar. The N atom must be considered sp3-hybridized.

The molecular orbitals for [NH(SO3)2]2− were calculated at

the HF/6-311+G** level. The highest-lying occupied molec-ular orbital (HOMO) is the lone-pair orbital on nitrogen. Oneof the lower-lying orbitals shows some π character betweennitrogen and sulfur. Two orbitals, HOMO(−20) andHOMO(−21), show σ bonds between nitrogen and sulfur.The bonds between the sulfur and oxygen atoms have both σand π characters.

2.2. Ions [N(SO3)2]3− and [P(SO3)2]

3−. The nitrogen andphosphorus atoms bonded to only two ligands are unusualstructures.Figure 2 shows the Lewis structures obeying the octet rules

for ions [N(SO3)2]3− and [P(SO3)2]

3−.

It is noted that the formal charge on nitrogen andphosphorus is −1, whereas that on the neighboring sulfuratoms is +2. Furthermore, the Lewis structures indicate thatboth nitrogen and phosphorus are divalent. Divalent nitrogenand phosphorus with two lone pairs are known fromorganometallic coordination chemistry, and such compoundsare commonly used in the design of new ligands.16−18

The structures of [N(SO3)2]3− and [P(SO3)2]

3− wereoptimized using MP2/6-311+G** calculations. The totalenergy for [N(SO3)2]

3− is −1300.09633 au, and for [P-(SO3)2]

3−, −1586.35125 au. Figure 3 shows the optimizedstructures of [N(SO3)2]

3− and [P(SO3)2]3−. It is noted that the

symmetry of [N(SO3)2]3− is C2, whereas that of [P(SO3)2]

3− isC2v. Imposing the C2v symmetry on [N(SO3)2]

3− results in aslightly higher energy and one imaginary vibrational frequencyof 33 cm−1. Therefore, this structure is not a stableconformation and the molecule in the C2v symmetry shouldbe considered as a transition state leading to the C2 geometry.Structural parameters obtained using MP2/6-311+G**

calculations for [N(SO3)2]3−and [P(SO3)2]

3− are shown inTable 2.

Figure 1. Structure of [NH(SO3)2]2− (C1) determined at the MP2/6-

311+G** level.

Table 1. Geometrical Parameters and Electrostatic Chargeson Each Atom in [NH(SO3)2]

2− Determined Using theMP2/6-311+G** Calculations

bond distances (Å) angles (deg)electrostaticcharges (e)

S1−N1 1.735 S1−N1−S2 127.82 N1 −0.79S2−N1 1.741 O1−S1−O2 114.73 S1 +1.68S1−O1 1.485 O1−S1−O3 111.95 S2 +1.66S1−O2 1.470 O2−S1−O3 114.48 O1 −0.86S1−O3 1.480 N1−S1−O1 100.45 O2 −0.76S2−O4 1.472 N1−S1−O2 105.19 O3 −0.81S2−O5 1.488 N1−S1−O3 108.65 O4 −0.78S2−O6 1.475 N1−S1−H1 106.54 O5 −0.85N1−H1 1.017 O4−S2−O5 113.84 O6 −0.78

O4−S2−O6 113.88 H1 +0.30N1−S2−O4 105.59N1−S2−O5 99.29N1−S2−O6 109.57N1−S2−H1 105.59

Figure 2. Lewis structures of [N(SO3)2]3− and [P(SO3)2]

3−.

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In [N(SO3)2]3−, the S−N bond lengths are 1.640 Å and the

average S−O bond length is 1.510 Å. For [P(SO3)2]3−, the S−P

bond lengths are 2.245 Å and the average S−O bond length is1.507 Å. The S−N−S angle is 127.53° for [N(SO3)2]

3−, andthe S−P−S angle for [P(SO3)2]

3− is 112.77°. The dihedralangle O1−S1−S2−O5 is derived to be 42.79° for [N(SO3)2]

3−

and 0.00° for [P(SO3)2]3−. This reveals that the two SO3

groups are twisted considerably in [N(SO3)2]3−, whereas

[P(SO3)2]3− retains the C2v geometry with the two SO3 groups

eclipsed to one another. This difference between the nitrogenand phosphorus compounds is presumably due to the longerS−P bonds, 2.245 Å, relative to the S−N bonds, 1.640 Å.Table 3 shows the electrostatic charges for the atoms in

[N(SO3)2]3− and [P(SO3)2]

3−. It is noted that the calculatedelectrostatic charge is −1.09e and −0.92e on nitrogen andphosphorus, respectively. For sulfur, the electrostatic chargesare +1.99e and +1.74e, respectively, for [N(SO3)2]

3− and[P(SO3)2]

3−. The average value of the charges on the oxygenatoms amounts to −0.98e for [N(SO3)2]

3− and −0.93e for[P(SO3)2]

3−.It is noted that the values of the electrostatic charges are very

much in line with the formal charges presented in the Lewisstructures in Figure 2.For [N(SO3)2]

3− and [P(SO3)2]3−, the molecular orbitals

were analyzed to elucidate the bonding properties. In Figure 4,selected molecular orbitals are shown for [N(SO3)2]

3− and[P(SO3)2]

3−. The molecular orbitals are derived using HF/6-311+G** calculations. The chosen orbitals for both ions are thehighest occupied molecular orbital (HOMO) and the lower-

lying molecular orbitals HOMO(−1), HOMO(−20), andHOMO(−21), respectively. The HOMO orbitals are π-typelone-pair orbitals for both ions. HOMO(−1) is a σ-type lone-pair orbital on the nitrogen and phosphorus atoms.HOMO(−20) and HOMO(−21) show σ bonds between thenitrogen or phosphorus and the sulfur atoms. A closerexamination of all of the valence orbitals reveals that no πcharacter is present between the nitrogen and sulfur atoms. Thesame applies for the bonds between phosphorus and sulfur.Thus, the molecular orbital analyses of [N(SO3)2]

3− and[P(SO3)2]

3− show that both ions are characterized by two lonepairs of electrons on the central nitrogen and phosphorusatoms. These lone pairs on the nitrogen and phosphorus atomsoccupy the two highest-lying molecular orbitals, HOMO andHOMO(−1). The electrostatic charges associated with N in[N(SO3)2]

3− and with P in [P(SO3)2]3− are close to −1e,

whereas the sulfur atoms in both ions have charges close to +2e.Surprisingly, no delocalization of electrons belonging to thenegatively charged atoms is observed.For further examination of the bond properties between

nitrogen and sulfur or phosphorus and sulfur, SPARTAN’1419

was used to determine the rotational barrier around the S−Nand S−P bonds calculated at the MP2/6-311+G** level.During these calculations, the dihedral angles of S1−N1−S2−O5 and S1−P1−S2−O5 were changed from 0 to 360° in stepsof 6°, while optimizing everything else. This resulted in arotational barrier of 2.89 kJ/mol for [N(SO3)2]

3− and 9.95 kJ/

Figure 3. Molecular structure of [N(SO3)2]3− (C2) (a) and

[P(SO3)2]3− (C2v) (b) determined at the MP2/6-311+G** level.

Table 2. Geometrical Parameters of [N(SO3)2]3− and [P(SO3)2]

3− Determined by the MP2/6-311+G** Calculations

[N(SO3)2]3− [P(SO3)2]

3−

bond distances (Å) angles (deg) bond distances (Å) angles (deg)

S1−N1 1.640 S1−N1−S2 127.53 S1−P1 2.245 S1−P1−S2 112.77S2−N1 1.640 O1−S1−O2 110.21 S2−P1 2.245 O1−S1−O2 110.54S1−O1 1.513 O1−S1−O3 109.00 S1−O1 1.514 O1−S1−O3 110.54S1−O2 1.506 O2−S1−O3 109.96 S1−O2 1.503 O2−S1−O3 110.49S1−O3 1.511 N1−S1−O1 103.36 S1−O3 1.503 P1−S1−O1 98.15S2−O4 1.511 N1−S1−O2 112.96 S2−O4 1.503 P1−S1−O2 113.27S2−O5 1.513 N1−S1−O3 111.12 S2−O5 1.514 P1−S1−O3 113.27S2−O6 1.506 O4−S2−O5 109.00 S2−O6 1.503 O4−S2−O5 110.54

O4−S2−O6 109.96 O4−S2−O6 110.49O5−S2−O6 110.21 O5−S2−O6 110.54N1−S2−O4 111.12 P1−S2−O4 113.27N1−S2−O5 103.36 P1−S2−O5 98.15N1−S2−O6 112.96 P1−S2−O6 113.27

Table 3. Electrostatic Charges (e) on the Atoms in the[N(SO3)2]

3− and [P(SO3)2]3− Ions As Derived by the MP2/

6-311+G** Calculations Using SPARTAN’1419

[N(SO3)2]3− (C2) [P(SO3)2]

3− (C2v)

atom electrostatic charge atom electrostatic charge

N1 −1.09 P1 −0.92S1 +1.99 S1 +1.74S2 +1.99 S2 +1.74O1 −1.01 O1 −0.93O2 −0.96 O2 −0.92O3 −0.98 O3 −0.92O4 −0.96 O4 −0.92O5 −1.01 O5 −0.93O6 −0.98 O6 −0.92

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mol for [P(SO3)2]3−. These barrier heights do not indicate any

significant π bonding.In view of these results, there is no evidence of the double-

bond character between nitrogen and sulfur in [N(SO3)2]3−

and the insignificant double-bond character between phospho-rus and sulfur in [P(SO3)2]

3−.Natural bond orbital (NBO) analyses have been carried out

on both ions. The results revealed that the S atoms are basicallysp3-hybridized with little contributions from the d polarizationfunctions. The d contributions in all sp3 hybrids of S in[N(SO3)2]

3− are 0.10. In the corresponding [P(SO3)2]3− ion,

the d orbital contributions amount to 0.09−0.10. Thus, theNBO analyses are consistent with single σ bonds between S andN, S and P, and also between all S and O atoms.Altogether, the analyses of the molecular orbitals and of the

electrostatic charges on the atoms in the ions indicate that theLewis structures of [N(SO3)2]

3− and [P(SO3)2]3− shown in

Figure 2 provide adequate representations of the two ions.2.3. Compounds CaNa[N(SO3)2]·3H2O and CaNa[P-

(SO3)2]·3H2O. In the investigation of the structure ofCaNa[N(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O, the posi-tioning of the cations, calcium and sodium, are based on thecrystal data derived from Fogh et al.1 With respect to water,various placements of the water molecules have been attemptedin relation to minimizing the total energy. The structures ofCaNa[N(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O are opti-mized by MP2/6-311G** calculations. Figure 5 shows theresulting structures for CaNa[N(SO3)2]·3H2O and CaNa[P-(SO3)2]·3H2O. The total energy is −2368.30795 and−2654.53101 au for CaNa[N(SO3)2]·3H2O and CaNa[P-(SO3)2]·3H2O, respectively.Table 4 shows the structural results obtained for CaNa[N-

(SO3)2]·3H2O in the MP2/6-311+G** calculations. Theexperimental values from Fogh et al.1 are also included. Table

5 shows the corresponding data obtained for CaNa[P(SO3)2]·3H2O.For CaNa[N(SO3)2]·3H2O, the S1−N1 bond length

obtained is 1.611 Å, whereas the S2−N1 bond length isslightly longer, 1.644 Å. The S−N bond lengths in the work ofFogh et al.1 are determined to be 1.602 and 1.604 Å,respectively, which are slightly shorter than the calculated S−N bond lengths. The S1−O1 and S2−O5 bond lengths arederived to be 1.465 and 1.479 Å, respectively. The average S−Obond length is 1.501 Å. The experimentally obtained1 S1−O1and S2−O5 bond lengths are 1.470 and 1.467 Å, respectively,which are in reasonable agreement with the calculated values.The remaining calculated S−O bonds are slightly longer thanthe ones obtained experimentally.1 The S−N−S angle isderived to be 120.10°, whereas the experimental value of the

Figure 4. Selected molecular orbitals for [N(SO3)2]3− (a) and [P(SO3)2]

3− (b) as derived by HF/6-311+G** calculations.

Figure 5. Lowest-energy structure for CaNa[N(SO3)2]·3H2O (a) andCaNa[P(SO3)2]·3H2O (b) determined by MP2/6-311+G** calcu-lations. Hydrogen bonds are indicated by the dotted lines.

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S−N−S angle is 118.10°. The dihedral angleO1−S1−S2−O5 is38.25°. Thus, the O1−S1−N1−S2−O5 atoms are far frombeing in plane, as was assumed in the experimental work.1,12,13

The present work represents the first theoretical inves-tigations on CaNa[P(SO3)2]·3H2O and the [P(SO3)2]

3− ion. Inaddition, no experimental work has been done on thesephosphorus systems.For CaNa[P(SO3)2]·3H2O, the calculated P1−S1 and P1−

S2 bond lengths are 2.163 and 2.180 Å, respectively. Thesebond lengths are shorter than the ones derived for [P(SO3)2]

3−,2.245 Å. The S1−O1 and S2−O5 bond lengths are 1.457 and

1.456 Å, respectively, which are shorter than those of thecorresponding bonds derived for [P(SO3)2]

3−, 1.514 Å. Theaverage S−O bond length is 1.508 Å. The S−P−S angle is96.343°, which is remarkably smaller than that identified for[P(SO3)2]

3−, 112.77°. The dihedral angle O1−S1−S2−O5 is49.63° for CaNa[P(SO3)2]·3H2O. This shows that [P(SO3)2]

3−

in the CaNa[P(SO3)2]·3H2O system no longer possesses theC2v symmetry.Tables 4 and 5 include the shortest distances between the

cations, calcium and sodium, and the oxygen atoms of theanions.

Table 4. Geometrical Parameters for CaNa[N(SO3)2]·3H2O Determined by the MP2/6-311+G** Calculations and Comparedwith the Corresponding Experimental Values by Fogh et al.1

bond distances (Å) ref 1 angles (deg) ref 1 distances (Å)

S1−N1 1.611 1.602(1) S1−N1−S2 120.10 118.10(7) Na+···N1 2.326S2−N1 1.644 1.604(1) O1−S1−O2 113.05 111.49(7) Na+···O5 2.468S1−O1 1.465 1.470(1) O1−S1−O3 115.98 109.75(7) Na+···O7 2.324S1−O2 1.529 1.463(1) O2−S1−O3 102.70 110.67(6) Ca2+···O2 2.414S1−O3 1.517 1.464(1) O1−S1−N1 105.88 109.57(6) Ca2+···O3 2.336S2−O4 1.525 1.465(1) O2−S1−N1 109.60 104.75(7) Ca2+···O4 2.251S2−O5 1.479 1.467(1) O3−S1−N1 109.59 110.51(7) Ca2+···O8 2.375S2−O6 1.490 1.465(1) O4−S2−O5 113.40 109.97(7) Ca2+···O9 2.371

O4−S2−O6 107.19 110.81(7)O5−S2−O6 113.96 111.80(7)O4−S2−N1 108.28 111.74(6)O5−S2−N1 102.32 110.81(7)O6−S2−N1 111.61 101.49(7)

hydrogen bonds (Å)

Omolecule···Hwater Omolecule···Owater angles, water-anion (deg)

O1···H1 1.823 O1···O7 2.795 O1···H1−O7 171.29O2···H3 1.864 O2···O8 2.614 O2···H3−O8 131.06O6···H5 1.686 O6···O9 2.663 O6···H5−O9 165.04

Table 5. Geometrical Parameters for CaNa[P(SO3)2]·3H2O Determined by MP2/6-311+G** Calculations

bond distances (Å) angles (deg) angles (deg) distances (Å)

S1−P1 2.163 S1−P1−S2 96.34 O4−S2−O5 111.50 Na+···O2 2.330S2−P1 2.180 O1−S1−O2 112.83 O4−S2−O6 104.13 Na+···O4 2.388S1−O1 1.457 O1−S1−O3 114.59 O5−S2−O6 114.80 Na+···O8 2.342S1−O2 1.552 O2−S1−O3 101.49 O4−S2−P1 110.07 Ca2+···O2 2.322S1−O3 1.519 O1−S1−P1 107.22 O5−S2−P1 106.42 Ca2+···O3 2.353S2−O4 1.546 O2−S1−P1 106.58 O6−S2−P1 109.93 Ca2+···O6 2.235S2−O5 1.456 O3−S1−P1 113.90 Ca2+···O7 2.369S2−O6 1.515 Ca2+···O9 2.363

hydrogen bonds (Å)

Omolecule···Hwater Omolecule···Owater angles, water-anion (deg)

O3···H1 1.984 O3···O7 2.641 O3···H1−O7 123.27O4···H3 1.827 O4···O8 2.650 O4···H3−O8 139.99O4···H5 1.615 O4···O9 2.594 O4···H5−O9 163.61

Table 6. Electrostatic Charges (e) on the Atoms of CaNa[N(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O Based on the StructuresDerived Using the MP2/6-311+G** Calculations

CaNa[N(SO3)2]·3H2O CaNa[P(SO3)2]·3H2O

atom electrostatic charge atom electrostatic charge atom electrostatic charge atom electrostatic charge

N1 −1.02 O1 −0.68 P1 −0.45 O1 −0.57S1 +1.65 O2 −0.88 S1 +1.22 O2 −0.82S2 +1.59 O3 −0.86 S2 +1.13 O3 −0.78Ca1 +1.69 O4 −0.83 Ca1 +1.69 O4 −0.75Na1 +0.90 O5 −0.73 Na1 +0.75 O5 −0.50

O6 −0.72 O6 −0.82

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For both CaNa[N(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O,three hydrogen bonds are observed, similar to the resultsobtained in the diffraction experiment of CaNa[N(SO3)2]·3H2O.

1 The calculated structural data obtained using MP2/6-311+G** calculations for CaNa[N(SO3)2]·3H2O resulted inoverall good agreement with the experimental data.The Owater−Hwater···Omolecule angle in the work by Fogh et al.1

ranges from 166 to 177°. The Omolecule···Owater distances rangefrom 2.731 to 2.898 Å, and the Hwater···Omolecule distances rangefrom 1.79 to 2.10 Å.1 These values compare reasonably with thecalculated values. According to the definition of Jeffrey,referenced by Steiner,20 the hydrogen bonds in CaNa[N-(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O are all of moderatestrength.Table 6 lists the electrostatic charges derived, on basis of the

structures obtained by the MP2/6-311+G** calculations, foreach of the atoms in CaNa[N(SO3)2]·3H2O and CaNa[P-(SO3)2]·3H2O.For the systems containing nitrogen, CaNa[N(SO3)2]·3H2O

and [N(SO3)2]3−, the electrostatic charge of nitrogen in the

bare ion, −1.09e, is numerically slightly larger than that ofnitrogen, −1.02e, in CaNa[N(SO3)2]·3H2O. The electrostaticcharges of the sulfur atoms in CaNa[N(SO3)2]·3H2O, +1.65eand +1.59e, respectively, are remarkably smaller than those of[N(SO3)2]

3−, +1.99e. For the oxygen atoms, the mostsignificant changes occur for O1 and O5. In CaNa[N(SO3)2]·3H2O, the charges are −0.68e and −0.73e, respectively, whereasboth of the corresponding charges in [N(SO3)2]

3− are −1.01e.Thus, in CaNa[N(SO3)2]·3H2O, relative to [N(SO3)2]

3−, thecharge is transferred from the oxygen atoms to the sulfur atomsand to the metal atoms, whereas the charge on nitrogen isalmost unchanged.For the systems containing phosphorus, CaNa[P(SO3)2]·

3H2O and [P(SO3)2]3−, there are significant changes in the

electrostatic charges. The electrostatic charge on phosphoruschanges from −0.92e in [P(SO3)2]

3− to −0.45e in CaNa[P-(SO3)2]·3H2O. For the sulfur atoms, the electrostatic charges in[P(SO3)2]

3− are +1.74e, and in CaNa[P(SO3)2]·3H2O, S1 andS2 have electrostatic charges of +1.22e and +1.13e, respectively.For both O1 and O5 in [P(SO3)2]

3−, the charges are −0.93e.These charges are reduced to −0.57e and −0.50e, respectively,in CaNa[P(SO3)2]·3H2O. The charges on the remaining Oatoms are numerically slightly reduced, that is, from −0.92e toapproximately −0.80e. In CaNa[P(SO3)2]·3H2O, relative to[P(SO3)2]

3−, the charge is transferred from the oxygen atomsto the sulfur atoms and to the metal atoms, whereas the chargeon phosphorus is reduced from −0.92 to −0.45. This ispresumably because the valence electrons of phosphorus areloosely bound relative to the valence electrons of nitrogen.NBO analyses have been carried out for CaNa[N(SO3)2]·

3H2O and CaNa[P(SO3)2]·3H2O. For both compounds, theresults revealed that the S atoms in the S−N, S−P, and S−O σbonds are basically sp3-hybridized with little contributions fromthe d polarization functions. The d contributions in all sp3

hybrids of S in CaNa[N(SO3)2]·3H2O are in the range of0.09−0.12. In CaNa[P(SO3)2]·3H2O, the corresponding dorbital contributions amount to 0.07−0.12. Thus, same as forthe [N(SO3)2]

3− and [P(SO3)2]3− ions, the NBO analyses are

consistent with single σ bonds between S and N, S and P, andalso between all S and O atoms.2.4. Raman Spectra. A CaNa[N(SO3)2]·3H2O crystal was

placed directly under a microscope, and a clear spot was usedfor recording. The provided crystal was brown and clear; the

brown areas were assigned to impurities, and it was proved tobe important to record the spectrum through a clear crystalarea. The recorded spectrum was obtained according to thespecifications given in the Materials and Methods section. Therecorded spectrum is corrected for the background and for theN2 band at 2331 cm−1.Figure 6 shows the experimentally obtained and calculated

spectra of CaNa[N(SO3)2]·3H2O.

The 0−1200 cm−1 range is mostly associated with theCaNa[N(SO3)2] moiety, whereas the 3000−4000 cm−1 range isassociated with the three crystal water molecules.Theoretical Raman spectra for CaNa[N(SO3)2]·3H2O and

CaNa[P(SO3)2]·3H2O were obtained by the HF/6-311+G**calculations using Gaussian’09.21 As suggested in the work ofMerrick et al.,22 an empirical scale factor of 0.9059 was used tocorrect the frequencies derived by the HF/6-311+G**calculations. The scaled frequencies are used to assign thebands obtained in the experimental spectra. The bendingmodes of the three water molecules in the salt were calculatedto be at a scaled value at around 1750 cm−1, but these modeswere not observed experimentally, probably because ofweakness. Table 7 lists the derived frequencies for CaNa[N-(SO3)2]·3H2O with assignments of the characteristic peaks.The S−N and S−O stretchings associated with the

[N(SO3)2]3− moiety are assigned to the bands in the 800−

1350 cm−1 range. Several authors13,23,24 have measured theRaman spectra of K3[N(SO3)2]·H2O. Their assignments of thesymmetric S−N and S−O stretchings at 794 and 1046 cm−1 arein good agreement with the scaled frequencies presented inTable 7.Figure 7 shows the spectra derived for [P(SO3)2]

3− andCaNa[P(SO3)2]·3H2O based on the HF/6-311+G** calcu-lations. The water bending modes in the spectrum, Figure 7b,are found between 1500 and 1750 cm−1.Table 8 lists the frequencies derived for the [P(SO3)2]

3− ion,as well as the frequencies for [P(SO3)2]

3− as part of

Figure 6. Raman spectra of two crystals of CaNa[N(SO3)2]·3H2O,obtained at room temperature. The spectrum obtained with 532.15nm excitation is shifted for ease of comparison to the spectrumobtained with 514.5 nm excitation. The top spectrum shows the scaledspectrum of CaNa[N(SO3)2]·3H2O, based on the HF/6-311+G**calculation and assuming Gaussian band shapes of half-height at half-width of 16 cm−1.

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CaNa[P(SO3)2]·3H2O with assignments of the characteristicpeaks.2.5. IR Spectra. Figure 8a shows two experimental IR

absorption spectra obtained for CaNa[N(SO3)2]·3H2O,dispersed in KBr powder and pressed under evacuation intorather clear disks.As for the Raman spectra, the range from approximately 400

to 1500 cm−1 is mostly associated with the CaNa[N(SO3)2]moiety itself and the range 1500−4000 cm−1 is mostlyassociated with the three molecules of crystal water, the lowmodes being due to bendings, the higher ones due to OHstretchings, the symmetric ones at lower values, and theasymmetric ones at somewhat higher wavenumbers. Thecalculated spectrum in Figure 8b is scaled by a factor of0.9059. This spectrum corresponds rather well with theexperimentally obtained spectra for CaNa[N(SO3)2]·3H2O,even though they probably also contain weak bands due to theoccurrence of overtone and combination transitions.

3. CONCLUSIONSThe results of the MP2/6-311+G** calculations on [NH-(SO3)2]

2− show that the arrangement around nitrogen is notplanar; therefore, nitrogen must be considered to be sp3-hybridized.

The results of MP2/6-311+G** calculations for [N-(SO3)2]

3− and [P(SO3)2]3− reveal that the central atom, that

is, N or P, is singly bonded to the S atoms in both ions. The Natom has a negative charge of −1.09e, whereas that of P is−0.92e. It is shown that N and P each has two lone pairs. Inaddition, nitrogen and phosphorus can be considered sp2-hybridized.NBO analyses show that the chemical bonds between N and

S as well as between P and S are single σ bonds and that boththe S and O atoms are sp3-hybridized with only a slightcontribution from the polarization d orbitals. All bondsbetween the S and O atoms are single σ bonds.Thus, the calculated structures of both ions are consistent

with the simple Lewis structures in which the nitrogen andphosphorus atoms are divalent, each with a negative charge andtwo lone pairs.The geometry of the isolated [N(SO3)2]

3− ion is notsignificantly different from that of the ion in CaNa[N(SO3)2]·3H2O. The geometry of [P(SO3)2]

3− changes significantlywhen the ion becomes part of CaNa[P(SO3)2]·3H2O, goingfrom the C2v symmetry to the C1 symmetry.When the anions, [N(SO3)2]

3− and [P(SO3)2]3−, become

part of CaNa[N(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O, thecharge is transferred from the oxygen atoms to the sulfur atomsand to the metal atoms. Whereas the charge on nitrogen hardlychanges, the charge on phosphorus is reduced significantly.This is presumably due to the valence electrons of phosphorusbeing loosely bound relative to the valence electrons ofnitrogen.The measured Raman and IR spectra associated with

[N(SO3)2]3− in CaNa[N(SO3)2]·3H2O have been assigned

by comparison with the calculated spectra. The vibrationalbands observed above 1500 cm−1 are mostly due to the watermolecules incorporated in the crystal structure, but other bandsare also observed probably due to the overtone andcombination transitions. In addition, the characteristic bandsassociated with the bare ion, [P(SO3)2]

3−, and with the ion in aCaNa[P(SO3)2]·3H2O crystal have been predicted andassigned.

Table 7. Experimentally Observed Spectral Bands forCaNa[N(SO3)2]·3H2O (in cm−1) Compared with theFrequencies Obtained by the HF/6-311+G** Calculations(800−1250 cm−1)a

experimentalcalculatedfrequencies

scaledfrequencies assignments

817 883 800 sym. stretch (S−N)991 1041 943 sym. stretch (S−O)1083 1141 1034 sym. stretch (S−O)1145 1177 1066 asym. stretch (S−N)1181 1305 1182 asym. stretch (S−N)1218 1344 1218 asym. stretch (S−O)

aAlso included are the scaled frequencies (cm−1).

Figure 7. (a) Scaled Raman spectrum of [P(SO3)2]3− based on the HF/6-311+G** calculation. (b) Scaled Raman spectrum of CaNa[P(SO3)2]·

3H2O based on the HF/6-311+G** calculations. Gaussian band shapes of half-height at half-width of 4 cm−1 have been assumed.

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4. MATERIALS AND METHODS

4.1. Experimental Methods. The CaNa[N(SO3)2]·3H2Ocrystals were received directly from the Enstedvaerk powerplant, Denmark. The Raman spectra of CaNa[N(SO3)2]·3H2Owere obtained using a DILOR XY 800 mm focal lengthmultichannel spectrometer with microentrance, laser excitation(532.15 and 514.5 nm, 400 mW) and cooled CCD detection.Filtration of Rayleigh scattering was done with notch filters, andcalibration was done with argon lines and Raman lines ofcyclohexane.25 Slits were set to 500 μm corresponding to aspectral resolution of approximately 6 cm−1. The spectra wereobtained by recording 12 frames in 120 s, two times.The spectra were not corrected for the small changes in the

instrument response versus wavenumber.IR absorption spectra were obtained from microcrystalline

samples dispersed in KBr on a PerkinElmer 1720 Fouriertransform instrument with a liquid-nitrogen-cooled Ge-diodedetector. The crystals were intensively mixed with KBr bygrinding in agate mortar and pressing the resulting powder intodisks under vacuum. The spectra were recorded in transmissionmode at 25 °C against a similar empty reference disk. Thespectral resolution was around 4 cm−1.

4.2. Computational Methods. All of the moleculesstudied were initially built using SPARTAN’14.19 For all ofthe calculations, Gaussian’09 was used.21 To obtain adequateconvergence, the optimization criteria in all calculationsperformed were chosen as tight, that is, with the maximumforce less than 0.000015 au and the maximum displacement lessthan 0.000060 au. The geometries of CaNa[N(SO3)2]·3H2Oand CaNa[P(SO3)2]·3H2O were optimized using restrictedMP2/6-311+G** calculations with the frozen core approx-imation. The molecular geometries of [N(SO3)2]

3− and[P(SO3)2]

3− were optimized imposing both C2 and C2v pointgroup symmetries. To ensure that the optimized energies arelocal minima on the potential energy surfaces, the vibrationalfrequencies were derived by diagonalization of the Hessianmatrix. The optimized structures of all investigated compoundswere identified as true minima because the Hessian matricesrevealed only positive eigenvalues. Furthermore, the Ramanspectra of [N(SO3)2]

3− and [P(SO3)2]3− as well as of

CaNa[N(SO3)2]·3H2O and CaNa[P(SO3)2]·3H2O were calcu-lated from the optimized structures determined using HF/6-311+G** calculations. The resulting frequencies were scaled bythe empirical factor of 0.9059.22 For CaNa[N(SO3)2]·3H2O,compared to that from the experimental data, the scalingconsisted in multiplication of the intensity values in Å4/AMUby 20 000, shifting by +20 000, and putting on a guessedGaussian profile of half-width at half-height of 16 cm−1. For

Table 8. Vibrational Frequencies of [P(SO3)2]3− and CaNa[P(SO3)2]·3H2O Derived by the HF/6-311+G** Calculation (in

cm−1)a

[P(SO3)2]3− CaNa[P(SO3)2]·3H2O

calculated frequencies scaled frequencies calculated frequencies scaled frequencies assignments

704 638 708 641 asym. stretch (S−P)719 651 734 665 sym. stretch (S−P)1033 936 1012 917 sym. stretch (S−O)1067 967 1076 975 sym. stretch (S−O)1173 1063 1103 999 asym. stretch (S−O)1190 1078 1142 1035 asym. stretch (S−O)1207 1093 1344 1218 asym. stretch (S−O)

aAlso included are the scaled frequencies.

Figure 8. (a) IR absorption spectra of two different KBr diskscontaining CaNa[N(SO3)2]·3H2O, at room temperature. (b)Absorption spectrum calculated based on the HF/6-311+G** resultsand using a Gaussian profile of half-width at half-height of 16 cm−1.The y axis values are obtained by calculating the percentage of theepsilon absorption values, followed by conversion to percenttransmission.

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[P(SO3)2]3− and CaNa[P(SO3)2]·3H2O bands, half-height at

half-width of 4 cm−1 has been assumed.The electrostatic charges on the atoms are defined as the

partial charges associated with each atom giving rise to anelectrostatic potential map that is identical within a chosenthreshold to that derived from the wave functions. Suchelectrostatic charges have been derived for all of thecompounds investigated using SPARTAN’14.19

■ AUTHOR INFORMATIONCorresponding Author*E-mail: shim@kemi.dtu.dk. Phone: +45 4525 5432. Fax: +454588 3136 (I.S.).ORCIDIrene Shim: 0000-0002-6584-5800NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe computations have been performed at the High Perform-ance Computer Center at the Technical University ofDenmark. We thank Folmer Fogh for gifting sample crystals.

■ REFERENCES(1) Fogh, F.; Hazell, A.; Rasmussen, S. E. Calcium SodiumIminodisulphonate Trihydrate, a Product of Flue-Gas Desulfurization.Acta Crystallogr., Sect. E: Struct. Rep. Online 2002, 58, i82−i84.(2) Rasmussen, S. E.; Grundvig, S.; Lundtoft, B.; Fogh, F. CrystalData for CaNa-Iminodisulphonate, Trihydrate. Powder Diffr. 2003, 18,230−232.(3) Claus, A.; Koch, S. Beitrage Zur Kenntniss Der Schwefelstick-stoffsauren. Ann. Chem. Pharm. 1869, 152, 336−350.(4) Berglund, E. Ueber Die Amidosulfonsaure. Ber. Dtsch. Chem. Ges.1876, 9, 252−256.(5) Divers, E.; Haga, T. Imidosulphonates. J. Chem. Soc., Trans. 1892,943−988.(6) Divers, E.; Haga, T. CX.Amidosulphonic Acid. J. Chem. Soc.,Trans. 1896, 69, 1634−1653.(7) Sisler, H.; Audrieth, L. F. Potassium Nitrilosulfonate. J. Am.Chem. Soc. 1938, 60, 1947−1948.(8) Jeffrey, G. A.; Jones, D. W. The Crystal Structure of PotassiumAminedisulphonate. Acta Cryst. 1956, 9, 283−289.(9) Cruickshank, D. W. J. Role of 3d-Orbitals in Pi-Bonds between(a) Silicon, Phosphorus, Sulphur, or Chlorine and (B) Oxygen orNitrogen. J. Chem. Soc. 1961, 5486−5504.(10) Cruickshank, D. W. J.; Jones, D. W. A Refinement of the CrystalStructure of Potassium Imidodisulphate. Acta Cryst. 1963, 16, 877−883.(11) Hodgson, P. G.; Moore, F. H.; Kennard, C. H. L.Redetermination of the Crystal Structure of Dipotassium Imidobis-(trioxosulphate) by Neutron Diffraction. J. Chem. Soc., Dalton Trans.1976, 1443−1445.(12) Barbier, P.; Parent, Y.; Mairesse, G. Crystal Structure ofK3N(SO3)2.H2O (I) and Refinement of the Crystal Structures ofK2NH(SO3)2 (II). Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst.Chem. 1979, B35, 1308−1312.(13) Hall, J. R.; Johnson, R. A.; Kennard, C. H. L.; Smith, G.; Skelton,B. W.; White, A. H. Crystal Structure and the Infrared and RamanSpectra of Potassium Nitridosulphonate Hydrate, K3[N(SO3)2].H2O.J. Chem. Soc., Dalton Trans. 1980, 1091−1097.(14) Roothaan, C. C. J. New Developments in Molecular OrbitalTheory. Rev. Mod. Phys. 1951, 23, 69−89.(15) Møller, C.; Plesset, M. S. Note on an Approximation Treatmentfor Many-Electron Systems. Phys. Rev. 1934, 46, 618−622.(16) Patel, D. S.; Bharatam, P. V. Divalent N(I) Compounds withTwo Lone Pairs on Nitrogen. J. Phys. Chem. A 2011, 115, 7645−7655.

(17) Ellis, B. D.; Dyker, C. A.; Decken, A.; Macdonald, C. L. B. TheSynthesis, Characterisation and Electronic Structure of N -Hetero-cyclic Carbene Adducts of P(I) Cations. Chem. Commun. 2005, 1965−1967.(18) Patel, D. S.; Bharatam, P. V. Novel (+)N(<-L)2 Species withTwo Lone Pairs on Nitrogen: Systems Isoelectronic to Carbodicar-benes. Chem. Commun. 2009, 1064−1066.(19) Spartan’14; Wavefunction, Inc.: Irvine, CA, 2014.(20) Steiner, T. The Whole Palette of Hydrogen Bonds REVIEWSThe Hydrogen Bond in the Solid State. Angew. Chem., Int. Ed. 2002,41, 48−76.(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.et al. Gaussian’09; Gaussian, Inc.: Wallingford CT, 2009.(22) Merrick, J. P.; Moran, D.; Radom, L. An Evaluation ofHarmonic Vibrational Frequency Scale Factors. J. Phys. Chem. A 2007,111, 11683−11700.(23) Siebert, H. Schwingungsspektren Einiger Derivate DerSchwefelsaure. Z. Anorg. Allg. Chem. 1957, 289, 15−28.(24) Touzín, J.; Ruzicka, A. Vibration Spectra of Imidodiselenate andImidodisulphate Anions. Collect. Czech. Chem. Commun. 1981, 46,2620−2632.(25) Berg, R. W.; Nørbygaard, T. Wavenumber Calibration of CCDDetector Raman Spectrometers Controlled by a Sinus Arm Drive.Appl. Spectrosc. Rev. 2006, 41, 165−183.

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